Mobile communications systems have been developed because there has been a need to free users to move away from fixed telephone terminals without impairing the availability of users. An example of an existing mobile communications system is the pan-European mobile communications system GSM (Global System for Mobile Communications). The General Packet Radio Service GPRS is one of the services of GSM Phase 2+. The GPRS service allows packet data transfer between mobile data terminals and external data networks, while the “original” GSM network allows circuit-switched communications. If the user is attached to a packet data service and to a circuit-switched service at the same time, he/she is connected to two different kinds of domains. The GPRS network architecture is illustrated in FIG. 1.
Simultaneously with the development of mobile communications systems, various services have also been introduced in mobile communications systems, such as value added services customized according to the user's location. Examples of location-specific value-added services include localized weather forecasts, entertainment programmes, timetables, navigation and locating a mobile user in an emergency. Additionally, the user's location can also be used for law-enforcement purposes. These services are called location services (LCS). Here the term ‘location service’ refers to the task of tracking the location of a mobile station in terms of geographical coordinates. This task is not necessary for routing calls. There are several known methods by which a mobile station can be located with reasonable precision. Details of the location procedure are not relevant to this invention, however.
Patent application WO 00/25545, which is incorporated herein by reference with its cross-references, discloses how to locate a target mobile station MS in systems comprising two different domains supporting the LCS. In WO 00/25545 an GMLC (Gateway Mobile Location Center), via which the location service requests are routed, receives as routing instructions for LCS the current address of a first network node providing location information in the first domain and the current address of a second network node providing location information in the second domain when both addresses are available. When the GMLC receives these two addresses, it determines the preferred type of domain via which the GMLC first tries to get location information on the target MS, and if the location information can not be obtained via the preferred domain, the GMLC tries the other domain.
A problem with this solution is that the GMLC may receive both addresses even when the target MS is not reachable via a domain since the entity providing the routing instructions does not know whether or not the target MS is reachable. Thus the GMLC may try to get location information first via a domain where the mobile station is not reachable. This leads to unnecessary delays and causes unnecessary signalling.
Similar problem exists also in the third generation mobile communications systems which also comprise a PC domain and a CS domain. Some of the systems comprise different access networks, such as a second generation access network (e.g. GSM) and a third generation access network. These different networks may also have different domains.